This invention relates to automatic gain ranging methods and apparatus and, more particularly, to systems wherein gain ranging is achieved through digital processing.
There are many examples where systems must be designed to cover an extremely broad range of measured values. For example, in a space vehicle docking maneuver, range data may be required for a distance of about three miles down to a few hundred feet where the final docking procedure takes over. At a range of a hundred feet a resolution of .+-.1/2 foot is required. In order to provide this resolution and range data up to three miles in the same system, a full fifteen bit code providing an excess of 32,000 increments would be required.
In the past automatic range control systems have been constructed as part of the analog measuring devices which have the effect of reducing the required digital word size. Typically, such systems include a device for determining when the measurement goes "off scale" and to then automatically change to a different scale. The fianl signal from such a device can include both the measured "on scale" value plus a code signal indicating which scale was used for obtaining the measurement.
The scale changing analog systems suffer the disadvantages of being slow, subject to drift, and poorly suited for integrated, low cost mass production. With such systems it is normally necessary to attempt a measurement and, if it goes "off scale", to then make a gain correction, another measurement, etc., until an "on scale" measurement is achieved. Also, in many of the prior analog gain changing systems using operational amplifiers, the initial "off scale" measurement would drive the operational amplifier into saturation causing further lost time while the amplifier comes out of saturation to prepare for the next measurement.